The present invention generally relates to semiconductor processing, and more particularly relates to a method of detecting process induced defects in a gate stack of flash memory devices.
Computers and electronic devices continue to become more and more powerful in terms of speed and expanding applications. These devices, as a result of being more powerful, also demand greater memory storage capacity to store data, such as digital audio, digital video, contact information, database data and the like. Also important for portable electronic and/or computer devices is the requirement that memory storage maintain its contents via a permanent storage device. Conventional desktop computer systems typically maintain information in permanent storage devices such as hard drives. However, portable devices, such as digital cameras, personal digital assistants, digital audio players, and the like, often do not employ those types of permanent storage devices for power, space, and weight reasons. Portable devices often must rely on their memory in order to store and maintain information or data.
Conventional desktop computer systems typically utilize volatile memory devices for temporary storage of programs and data, due to its high speed operation but rely on permanent storage devices such as hard drives for permanent or long term data storage. Volatile memory devices, such as, for example, random access memory (RAM), DRAM, SRAM and the like, require regular refresh cycles to maintain their information and fail to maintain that information once power is lost for a substantial amount of time. With respect to portable devices, volatile memory devices may meet the speed requirements, but generally fail to be feasible for portable devices due to volatile memory""s inability to maintain data for substantial periods of time without power and/or refresh cycles.
Another type of memory device, in addition to volatile memory, is non-volatile memory. These devices maintain their information and/or data for substantial periods of time without refresh cycles and/or power and typically provide sufficient amounts of speed in operation. Because of these characteristics, non-volatile memory devices are often employed in portable electronic and/or computer devices.
A widely utilized type of non-volatile memory device is flash memory. Flash memory devices achieve relatively high speeds of operation and are able to maintain their data without power and/or refresh cycles. Flash memory devices are programmed and erased by pulses that place charges in selected memory cells or drain charges away. Additionally, blocks and arrays can be erased at a single time by an erase operation such as a channel erase.
Erase operations for flash memory devices are typically performed at greater voltage levels than those used in standard non-flash memory devices. Such erase operations can cause operation of flash memory devices and the devices themselves to be susceptible to extrinsic defects, introduced in the fabrication process, that would not generally be a problem for other types of memory devices. Costly and time intensive testing can be performed to identify devices with extrinsic defects, but such testing is not feasible and, therefore, the extrinsic defects are not necessarily identified. What is needed is a cost and time efficient method of identifying individual flash memory devices having extrinsic defects.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor to delineate the scope of the invention. Its primary purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention relates to a method of detecting process induced defects, also referred to as extrinsic defects, in the gate stack of flash memory devices and also to methods of fabricating flash memory devices that incorporate a high voltage stress test. By detecting these extrinsic defects, further processing of defective devices, meaning devices that have extrinsic defects, can be avoided resulting in significant cost and time savings. Traditional test mechanisms can fail to identify memory cells/devices with extrinsic defects as being defective.
The present invention applies a relatively high voltage across a gate stack of a flash memory cell for a certain period of time. Then, the polarity of the applied voltage is reversed and is again applied across the gate stack for another certain period of time. The voltage applied is greater than a channel erase voltage utilized for the memory cell during normal device operation. This applied voltage causes extrinsic defects to become amplified, for example, at interfaces of oxide/insulator layers of the gate stack. The memory cell is then tested (e.g., via a battery of tests) in order to determine if the memory cell is defective. If the cell is defective (e.g., fails the test), it can be assumed that substantial extrinsic defects were present in the memory cell and have been amplified resulting in the test failure. If the cell passes the test, it can be assumed that the memory cell is substantially free from extrinsic defects. Defective memory cells/devices can be marked or otherwise indicated as being defective thereby avoiding further parametric and/or functional testing of such defective devices.